U.S. patent application number 16/812649 was filed with the patent office on 2020-11-19 for gas field ionization source.
The applicant listed for this patent is Hitachi, Ltd.. Invention is credited to Shinichi MATSUBARA.
Application Number | 20200365360 16/812649 |
Document ID | / |
Family ID | 1000004699674 |
Filed Date | 2020-11-19 |
United States Patent
Application |
20200365360 |
Kind Code |
A1 |
MATSUBARA; Shinichi |
November 19, 2020 |
GAS FIELD IONIZATION SOURCE
Abstract
A gas field ionization source for forming an electric field for
ionizing gas comprises: an emitter tip having a tip end; an
extraction electrode facing the emitter tip and having an aperture
at a position distant therefrom; a gas supply means for supplying
the gas in the vicinity of the emitter tip; a vacuum partition made
of a metal having a hole; and a high voltage power source for
applying voltage between the emitter tip and the extraction
electrode. The hole is constructed so that the tip end of the
emitter tip can pass therethrough and the vacuum partition has a
micro protrusion, around the hole, protruding toward a side of the
extraction electrode.
Inventors: |
MATSUBARA; Shinichi; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi, Ltd. |
Tokyo |
|
JP |
|
|
Family ID: |
1000004699674 |
Appl. No.: |
16/812649 |
Filed: |
March 9, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 1/46 20130101; H01J
7/24 20130101 |
International
Class: |
H01J 7/24 20060101
H01J007/24; H01J 1/46 20060101 H01J001/46 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 2019 |
JP |
2019-091313 |
Claims
1. A gas field ionization source for forming an electric field for
ionizing gas, comprising: an emitter tip having a tip end; an
extraction electrode facing the emitter tip and having an aperture
at a position distant therefrom; a gas supplier for supplying the
gas in the vicinity of the emitter tip; a vacuum partition made of
a metal having a hole, the hole being constructed so that the tip
end of the emitter tip can pass therethrough; and voltage applier
for applying voltage between the emitter tip and the extraction
electrode, wherein the vacuum partition has a convex structure,
around the hole, protruding toward a side of the extraction
electrode.
2. The gas field ionization source according to claim 1,
constructed so that the gas supplier is electrically connected to
the emitter tip or an electric potential thereof is equal to that
of the emitter tip.
3. The gas field ionization source according to claim 1, comprising
an adjustor for adjusting a relative positional relationship
between the emitter tip and the hole of the vacuum partition.
4. The gas field ionization source according to claim 1, wherein:
an external shape of the convex structure is a cylindrical shape;
and a ratio of an external diameter of the convex structure to an
axial length thereof is equal to or less than 1.22.
5. The gas field ionization source according to claim 2,
comprising: an additional gas supplier for supplying the gas to the
gas supplier; an insulation tube for connecting the gas supplier
and the additional gas supplier; and an evacuator for evacuating an
inside of the insulation tube.
6. The gas field ionization source according to claim 2, wherein:
the gas field ionization source comprises a vacuum container
containing the emitter tip, the extraction electrode and the vacuum
partition; the gas field ionization source is constructed so that
the gas supplier is electrically connected to the emitter tip or an
electric potential thereof is equal to that of the emitter tip; and
the gas supplier is not contained in the vacuum container.
7. The gas field ionization source according to claim 6, wherein:
the gas supplier is contained in a cover made of a metal; and the
cover is electrically connected to the vacuum container or is
grounded.
8. The gas field ionization source according to claim 3, wherein:
the adjustor comprises a supporter for supporting the emitter tip;
and a screw channel is formed on the supporter.
9. The gas field ionization source according to claim 8, wherein an
axis of the screw channel passes through the hole of the vacuum
partition.
10. The gas field ionization source according to claim 1, wherein a
distance between the convex structure and the extraction electrode
is equal to or less than 1.5 mm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from Japanese patent
application JP 2019-091313 filed on May 14, 2019, the entire
content of which is hereby incorporated by reference into this
application.
BACKGROUND
Technical Field
[0002] The present invention relates to a gas field ionization
source.
[0003] Structure of a surface of a sample can be observed by
converging an electron beam via an electromagnetic field lens,
irradiating this for scanning on the sample and detecting secondary
electrons/charged particles emitted from the sample. This is called
a Scanning Electron Microscope. On the other hand, the structure of
the surface of the sample can also be observed by converging an ion
beam via an electromagnetic field lens, irradiating this for
scanning on the sample and detecting secondary charged particles
emitted from the sample. This is called a Scanning Ion Microscope
(SIM hereinafter).
[0004] One type of ion sources used for the SIM is a Gas Field
Ionization Source (hereinafter this may referred to as GFIS). A
GFIS applies a high voltage to a metal emitter tip, preferably the
radius of curvature in its tip is equal to or less than about 100
nm, focuses electric field to the tip, introduces gas (ionization
gas) around the tip, ionizes the gas molecules by the electric
field and extracts an ion beam. Hereinafter, the gas is referred to
as "ionization gas" including the gas before being ionized, and
this does not always mean "gas including ions" or "ionized
gas".
[0005] In a SIM using a GFIS (hereinafter GFIS-SIM), apparatuses
using helium ion beam or neon beam are mainstream. This is because
an ion beam emitted from a GFIS has a narrower energy band and
smaller light source size compared with a liquid-metal ion source
or an ion source utilizing plasma phenomenon so that the beam can
be converged to a smaller size.
BACKGROUND ART
[0006] JP 2016-76431A discloses a construction example comprising a
container filled with material gas, a needle-shaped electrode
arranged within the container and a micro aperture provided at an
outer wall of the container wherein a pressure inside the container
is higher than the outside, the material gas is ejected from inside
the container to outside through the micro aperture, around the
micro aperture is an aperture electrode, the needle-shaped
electrode and the aperture electrode are in the same electric
potential, the material gas is ionized by electric field ionization
by applying a strong electric field at the tip of the needle-shaped
electrode by the electric potential difference between the
needle-shaped electrode and the aperture electrode and an external
electrode arranged at the outside of the container, and extracts
the ions.
[0007] JP 2013-8471A discloses an example of gas ion source which
generates an ion beam by applying a voltage between an electric
field ionization electrode and an external electrode so that
electric field is formed in the vicinity of the tip of the electric
field ionization electrode and electric-field ionizing material gas
ejected from a micro aperture of an ion source container, wherein a
portion of a surface of the ion source container, a portion or an
entire portion of a surface of the external electrode or an entire
portion of the external electrode located in a region in the
vicinity of the tip of the electric field ionization electrode is
constructed by a thin film of a second electron emission
suppressing material.
SUMMARY
[0008] However, conventional techniques have a problem that the
emitter tip has to be cooled to a low temperature in order to
maintain a sufficient ion beam intensity.
[0009] Temperatures required for cooling differ depending on the
ions to be used, but generally, gases having lower condensation
temperatures (e.g. hydrogen or helium) require lower cooling
temperatures for maintaining a sufficient intensity. For cooling of
the emitter tip, there are a method using a coolant such as liquid
nitrogen or liquid helium and a method using a mechanical
refrigerator e.g. of Gifford-McMahon type.
[0010] The method using coolant requires a high running cost and
user's labor for supplying the coolant. On the other hand, the
method using the mechanical refrigerator requires a high initial
cost. Also, in the both methods, the emitter tip vibrates upon
cooling operation, which leads to deterioration of a quality of a
scanned ion image or accuracy upon working by the ion beam.
[0011] The invention provides a gas field ionization source wherein
the emitter tip does not have to be cooled or is operable with a
weaker cooling.
[0012] A gas field ionization source related to the present
invention is a gas field ionization source for forming an electric
field for ionizing gas, comprising: [0013] an emitter tip having a
tip end; [0014] an extraction electrode facing the emitter tip and
having an aperture at a position distant therefrom; [0015] a gas
supplier for supplying the gas in the vicinity of the emitter tip;
[0016] a vacuum partition made of a metal having a hole, the hole
being constructed so that the tip end of the emitter tip can pass
therethrough; and [0017] voltage applier for applying voltage
between the emitter tip and the extraction electrode, [0018]
wherein the vacuum partition has a convex structure, around the
hole, protruding toward a side of the extraction electrode.
[0019] According to a gas field ionization source related to the
present invention, the emitter tip does not have to be cooled or
can be operable with a weaker cooling.
[0020] Thus, for example, a gas field ionization source in a
GFIS-SIM can be operated without using any coolant or mechanical
refrigerator. An exemplary effect is that the initial introduction
cost and the running cost of an apparatus can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagram showing a construction of a gas field
ionization source related to a first embodiment of the present
invention;
[0022] FIG. 2 is a schematic cross section explaining a
construction of a vacuum partition of FIG. 1; and
[0023] FIG. 3 is a photograph explaining a construction of a vacuum
partition of FIG. 1;
[0024] FIG. 4 is a diagram showing dimensions related to a micro
protrusion of FIG. 1;
[0025] FIG. 5 is a diagram showing a construction for supporting an
emitter tip of FIG. 1;
[0026] FIGS. 6A and 6B are diagrams showing an effect of an
experiment example according to an experimental example of the
present invention; and
[0027] FIG. 7 is a diagram showing a construction of a gas field
ionization source related to a second embodiment of the present
invention.
DETAILED DESCRIPTION
[0028] The inventor studied a technique for improving an intensity
of the gas field ionization source to a practical level without
using any means for cooling the emitter tip.
[0029] In an embodiment, a vacuum partition is provided between the
emitter tip and an extraction electrode, a hole is provided at the
vacuum partition for projecting a tip end of the emitter tip toward
the side of the extraction electrode, ionization gas is supplied
from the side of a base of the emitter tip, and the ionization gas
is evacuated from the extraction electrode side, so that a gas
pressure is raised in the vicinity of the tip end of the emitter
tip.
[0030] In an embodiment, the electric field for ionizing the
ionization gas around the tip end of the emitter tip can be
generated without applying a large electric potential difference
(e.g. of equal to or more than 30 kV) between the emitter tip and
the extraction electrode by employing a convex structure wherein
the hole is slightly elevated around the side of extraction
electrode.
[0031] In an embodiment, the gas pressure of the ionization gas
supplied to the side of the emitter tip can be raised without a
risk of electric discharge by equalizing an electric potential of
an ionization gas supply means (e.g. a gas cylinder) with an
electric potential of the emitter tip.
[0032] In an embodiment, a balance can be adjusted between the
electric field generated in the vicinity of the tip end of the
emitter tip and the pressure of the ionization gas supplied around
the emitter tip by providing a mechanism for adjusting a relative
position between the emitter tip and the extraction electrode.
[0033] The present invention will be explained below in accordance
with embodiments referring to the drawings. However, the present
invention can be implemented in any construction other than the
embodiments described below within a range not departing from ideas
of the present invention defined in the appended claims.
First Embodiment
[0034] A gas field ionization source related to a first embodiment
of the present invention will be described referring to FIGS. 1, 2,
3, 4 and 5.
[0035] A gas field ionization source 1 comprises an emitter tip 11
(also referred to as an emitter electrode), an extraction electrode
13, a vacuum chamber 17 (vacuum container), an evacuation means 16,
a gas cylinder 35, high voltage power sources 111 and 112 and a
micro channel plate 5, and forms an electric field for ionizing a
gas (ionization gas).
[0036] The emitter tip 11 has a tip end. The tip end means, for
example, an end at the side of the extraction electrode 13. The tip
end is preferably formed in a shape that can generate a strong
electric field within a small range, and for example of a shape
described as "needle-shaped". The extraction electrode 13 has an
extraction electrode hole 131 (aperture). The extraction electrode
hole 131 is provided distant from the emitter tip 11 (precisely,
the tip end thereof) and facing this.
[0037] The vacuum chamber 17 contains the emitter tip 11 and the
extraction electrode 13. The evacuation means 16 is connected to
the vacuum chamber 17. The gas cylinder 35 supplies the ionization
gas to an inside of the vacuum chamber 17.
[0038] The high voltage power source 111 applies a voltage between
the emitter tip 11 and a ground potential. In order for this, an
electrically conductive member (not shown) may be placed between
the high voltage power source 111 and the emitter tip 11. Also, the
high voltage power source 112 applies a voltage between the
extraction electrode 13 and the ground potential. Thus, the high
voltage power sources 111 and 112 act as means for applying a
voltage between the emitter tip 11 and the extraction electrode 13,
thereby forming an electric field for ionizing (e.g. into positive
ions) a gas in the vicinity of the tip end of the emitter tip
11.
[0039] The high voltage power sources 111 and 112 may be
constructed so that the electric potentials of emitter tip 11 and
the extraction electrode 13 are independently controllable. This
allows controlling voltages for accelerating an ion beam, forming
the electric field and extraction. However, it is possible to use
only one of the high voltage power sources 111 and 112 so that the
other is omitted (in that case, the omitted side may be always at
the ground potential).
[0040] The gas field ionization source 1 first applies a high
voltage between the emitter tip 11 and the extraction electrode 13
in order to emit an ion beam 15 from the emitter tip 11. The
application of the high voltage focuses a strong electric field in
the vicinity of the tip end of the emitter tip 11. The strength of
the electric field formed in the vicinity of the tip end is
controlled to strength sufficient for ionizing the ionization gas
(e.g. ionizing hydrogen gas into positive ions). At this state,
using the gas cylinder 35 to introduce the ionization gas into the
vacuum chamber 17 (in particular, in the vicinity of the emitter
tip 11) emits an ion beam from the tip end of the emitter tip
11.
[0041] The ion beam 15 strikes a sample to be observed and
secondary electrons or secondary charged particles are emitted from
the sample. The emitted secondary electrons or secondary charged
particles are detected at the micro channel plate 5.
[0042] An insulator 132 may be placed between the emitter tip 11
(or a portion electrically connected to the emitter tip 11) and the
extraction electrode 13 (or a portion electrically connected to the
extraction electrode 13). In this way, a higher voltage can be
applied between the emitter tip 11 and the extraction electrode
13.
[0043] Also, a movable member (e.g. bellows 19) may be placed
between the emitter tip 11 (or a portion electrically connected to
the emitter tip 11) and the extraction electrode 13 (or a portion
electrically connected to the extraction electrode 13). In this
way, a positional relationship between the emitter tip 11 and the
extraction electrode 13 can be adjusted. The movable member is
constructed so that, for example, a distance between the tip end of
the emitter tip 11 and the extraction electrode hole 131.
[0044] Further, the gas field ionization source 1 may be
constructed so that various ionization gases can be introduced in
the vicinity of the emitter tip 11. For example, not only the
single gas cylinder 35 but also another gas cylinder may be
comprised. Also, if a plurality of gas cylinders are used, the gas
cylinders may supply respectively different types of ionization
gas. As types of the ionization gas, for example, helium, neon,
argon, krypton, xenon, nitrogen, oxygen, hydrogen, etc. can be
used.
[0045] It is constructed that the ionization gas is first supplied
from the gas cylinder 35 to a gas supply means 31 via a flow rate
adjusting means 37, an insulation tube 38 and a valve 33. The gas
supply means 31 may be a metal (or electrically conductive)
airtight container. The gas supply means 31 may be a gas cylinder.
The gas supply means 31 supplies the ionization gas in the vicinity
of the emitter tip 11 (in particular, in the vicinity of the tip
end of the emitter tip 11).
[0046] Here, the gas cylinder 35 can be considered to be an
additional gas supply means for supplying gas to the gas supply
means 31. An insulation tube 38 connects the gas supply means 31
and the gas cylinder 35 as an insulating pipe. Because of this
construction, the gas supply means 31 and the gas cylinder 35 are
in fluid communication indirectly while being electrically
insulated.
[0047] Also, after the ionization gas is supplied from the gas
cylinder 35 to the gas supply means 31, the ionization gas residing
within a flow path (including the insulation tube 38) may be
evacuated. For example, after closing the valve 33 and the flow
rate adjusting means 37, a valve 36 may be opened and the
ionization gas between the valve 33 and the flow rate adjusting
means 37 may be evacuated by the vacuum pump 34. Thus the vacuum
pump 34 evacuates the inside of the insulation tube 38.
[0048] The ionization gas can be prevented from electric discharge
by thus evacuating the flow path. In particular, the inside of the
insulation tube 38 is evacuated, so the path of electrical
discharge can be eliminated entirely.
[0049] The gas supply means 31 is contained in the vacuum chamber
17, in a manner similar to the emitter tip 11. The gas field
ionization source 1 may comprise an emitter tip supporting
component 113, an emitter tip positioning component 114, a vacuum
partition 12 and a flow rate adjusting means 32. The vacuum
partition 12 is contained in the vacuum chamber 17.
[0050] The emitter tip supporting component 113, the emitter tip
positioning component 114, the vacuum partition 12, the flow rate
adjusting means 32 and the gas supply means 31 may all be
constructed from metal (or electrically conductive) members or
materials and they all may be electrically connected. In this way,
electric potentials of all these constructional elements will be
equal to an electric potential present at the emitter tip 11 by the
high voltage power source 111. That is, the gas supply means 31 is
electrically connected to the emitter tip 11 or has the electrical
potential equal to the emitter tip 11.
[0051] Further, in that case, the insulation tube 38 may be placed
between these constructional elements (e.g. the gas supply means
31) and the vacuum chamber 17 so that they are electrically
insulated. According to such a construction, the emitter tip 11 and
the gas supply means 31 will have an equal electric potential, so
there will be no risk of electric discharge of the gas between the
emitter tip 11 and the gas supply means in principle even if the
ionization gas is present therebetween at a high gas pressure. That
is, the pressure of the ionization gas introduced around the
emitter tip 11 can be higher, allowing a greater electric current
for the ion beam 15.
[0052] Also, in a case wherein the flow rate adjusting means 32 and
the valve 33 are contained in the vacuum chamber 17 as the present
embodiment, an adjusting mechanism for controlling the flow rate
adjusting means 32 and the valve 33 may be provided outside of the
vacuum chamber 17. For example, an atmosphere-side adjusting
mechanism 330 is provided and connected to the valve 33 (or the
flow rate adjusting means 32) by a connection mechanism 331. The
connection mechanism 331 may be a power transmission mechanism or
an information transmission means. If the connection mechanism 331
is a power transmission mechanism, it can be constructed by an
electrically insulating member. If the connection mechanism 331 is
an information transmission means, it can be constructed to
communicate an instruction issued from the atmosphere-side
adjusting mechanism 330 to the valve 33 (or the flow rate adjusting
means 32) in a remote manner by using an electric wave or the
like.
[0053] In the present embodiment, an electrical current of the ion
beam 15 is increased by introducing the ionization gas at a high
gas pressure in the vicinity of the emitter tip 11. Here,
generally, in a gas field ionization source, as the gas pressure
gets higher, the degradation in convergence performance of the ion
beam 15 gets greater due to collision between the ion beam 15 and
gas molecules.
[0054] A ratio of the ion beam 15 being scattered increases as the
gas pressure rises. This ratio is expressed by an average value of
distance wherein an ion flies in a straight line without colliding
with any gas molecule (i.e. mean free path). It is evident that the
mean free path decreases as the gas pressure rises. If the mean
free path is shorter than the distance of the ion beam 15 from the
tip end of the emitter tip 11 to a target sample, the ion beam 15
becomes difficult to converge due to an effect of the above
scattering. That is, even if there is no risk of electric
discharge, instead, there will be an upper limit for the pressure
in the introduced ionization gas due to the effect of the
scattering.
[0055] Some conventional gas field ionization sources have a
construction for reducing the effect of the above scattering by
heightening the vacuum in the path of the ion beam by using
differential pumping wherein evacuation is constructed in
multi-stage. Specifically, a highly airtight container enclosing
the emitter tip and the extraction electrode is used for sealing,
the ionization gas is introduced therein, the ionization gas is
evacuated only from an extraction electrode hole so that the
ionization gas pressure is higher only around the emitter tip,
thereby the effect of the above scattering problem would be
reduced.
[0056] Such differential pumping can also be employed in the
present embodiment. The differential pumping can for example be
realized by providing a vacuum partition 12 around the emitter tip
11.
[0057] A construction of the vacuum partition 12 will be explained
using FIGS. 2 and 3. FIG. 2 is a schematic cross section and FIG. 3
is a photograph of a portion of FIG. 2 taken in a direction
approximately same as FIG. 2. The vacuum partition 12 is made of a
metal (or is electrically conductive). The vacuum partition 12 is
not an airtight partition which maintains the vacuum strictly but
has a hole 125 whereby an air pressure difference can be maintained
stationary if the ionization gas is supplied at a constant flow
rate. The hole 125 is constructed so that the tip end of the
emitter tip 11 (or a portion of a specific length including the tip
end) can pass therethrough. In the example of FIG. 2, an internal
diameter of the hole 125 is greater than an external diameter of
the emitter tip 11 (more precisely, an external diameter of a
portion of a specific length including, or in the vicinity of, the
tip end of the emitter tip 11) so that the emitter tip 11 can pass
therethrough.
[0058] The emitter tip 11 is placed passing through the hole 125 so
that the base side is at the high-pressure side, i.e. the side of
the flow rate adjusting means 32, and the tip end side is at the
low-pressure side, i.e. the side of the extraction electrode 13,
with respect to the vacuum partition 12. As indicated by the
reference numeral 117 in FIG. 2, the tip end of the emitter tip 11
protrudes from the hole 125 of the vacuum partition 12 toward the
side of the extraction electrode 13. The vacuum partition 12 allows
raising the pressure at the tip end of the emitter tip 11
locally.
[0059] Here, if the vacuum partition 12 were formed of a dielectic
material, it might be possible to further raise an electric field
strength around the tip end so that more complete ionization would
be performed. However, the dielectic material around the emitter
tip 11 would be charged so that the electric field strength would
be unstable. On the other hand, if the vacuum partition 12 is
formed of a metal or an electrically conductive material as in the
present embodiment, the electric field is shielded so that it would
be difficult to form a sufficient electric field strength in the
vicinity of the tip end of the emitter tip 11 depending on the
shape of the vacuum partition 12.
[0060] Accordingly, in the present embodiment, a micro protrusion
121 is provided at the vacuum partition 12 in order to suppress an
effect of shielding the electric field. The micro protrusion 121 is
an example of a convex structure protruding toward the side of the
extraction electrode 13 around the hole 125 of the vacuum partition
12. Note that, as shown in FIG. 2, the tip end of the emitter tip
11 protrudes further toward the side of the extraction electrode 13
from an end of the micro protrusion 121.
[0061] Thus, the emitter tip 11 protrudes (reference numeral 117)
from the hole 125 of the vacuum partition 12 through the micro
protrusion 121 toward the side of the extraction electrode 13, so
the electric field around the tip end of the emitter tip 11 is not
shielded very strictly, so the electric field formed at the tip end
of the emitter tip 11 will be stronger and an electric field
strength sufficient to ionize the ionization gas can be generated
easily.
[0062] A shape of the micro protrusion 121 may be constructed in a
cylindrical shape wherein the axial direction is a direction in
which the ion beam 15 is emitted. Also, the micro protrusion 121
may be formed to surround the emitter tip 11 at around the hole 125
of the vacuum partition 12.
[0063] FIG. 4 shows dimensions related to the micro protrusion 121.
The dimensions of the micro protrusion 121 may be designed in any
manner, but for example can be constructed so that a ratio of an
external diameter 123 of the micro protrusion 121 (the diameter, if
an external shape of the micro protrusion 121 is a cylindrical
shape) to a height 122 of the micro protrusion 121 (i.e. an axial
length) (that is, the ratio is a value of the external diameter
123/the height 122) is equal to or less than 1.22. Such a value can
realize a better balance between the ionization gas pressure and
the electric field strength.
[0064] As specific ranges for the dimensions in the shape, the
height 122 of the micro protrusion 121 can be equal to or more than
0.5 mm and the external diameter 123 of the micro protrusion 121
can be equal to or less than 0.61 mm. Also, in this case, an
internal diameter of the hole 125 of the micro protrusion 121 (the
diameter, if an internal shape of the micro protrusion 121 is a
cylindrical shape) may be equal to or less than 0.21 mm.
[0065] Also, a distance 124 between the micro protrusion 121 and
the extraction electrode 13 may for example be equal to or less
than 1.5 mm. By reducing the distance 124, the electric field
strength around the emitter tip 11 can be maintained in a high
value.
[0066] FIG. 5 shows a construction for supporting the emitter tip
11. The gas field ionization source 1 related to the present
embodiment comprises an adjusting mechanism for adjusting relative
positional relationship between the emitter tip 11 and the hole 125
of the vacuum partition 12 (e.g. a positional relationship in an
axial direction). In the example of FIG. 5, the adjusting mechanism
comprises an emitter tip supporting component 113 and an emitter
tip positioning component 114.
[0067] The emitter tip supporting component 113 fixes and supports
the emitter tip 11. Also, the emitter tip supporting component 113
is supported by the emitter tip positioning component 114. The
emitter tip positioning component 114 is fixed to the vacuum
partition 12 and its positional relationship with respect to the
hole 125 is also fixed. The emitter tip positioning component 114
comprises a function for adjusting relative positional relationship
between the emitter tip 11 and the hole 125 of the vacuum partition
12 freely by adjusting its own relative positional relationship
with respect to the emitter tip supporting component 113.
[0068] In the example of FIG. 5, adjustment of the positional
relationship is realized by a screw mechanism. That is, a screw
channel 115a is formed on an external circumference of the emitter
tip supporting component 113 and a corresponding screw channel 115b
is formed on in internal circumference of the emitter tip
positioning component 114. The emitter tip 11 moves in an axial
direction with respect to the hole 125 when the emitter tip
supporting component 113 and the emitter tip positioning component
114 fit and move in rotation with respect to each other along
respective screw channels 115a and 115b.
[0069] Such a construction allows easy adjustment of the distance
between the emitter tip 11 and the extraction electrode 13 (or a
center of the extraction electrode hole 131) merely by rotating the
emitter tip supporting component 113 along the screw channels 115a
and 115b. By using such an adjusting mechanism, the position of the
emitter tip 11 (in particular, the position of the tip end) can be
adjusted, so a better balance can be realized between pressure
distribution of the ionization gas and the electric field at the
tip end of the emitter tip 11. Also, by realizing the adjusting
mechanism using in particular the screw channels, the construction
of the adjusting mechanism can be comparatively simple.
[0070] It may be constructed so that a rotation axis 116 of the
screw channel 115a passes through the hole 125 of the vacuum
partition 12. Also, it may be constructed so that the rotation axis
116 of the screw channel 115a is aligned with an axis of the
emitter tip 11. In this construction, a lateral position (i.e. a
position in a direction perpendicular to the axis) of the emitter
tip 11 can be fixed beforehand, so positioning operation upon use
can be performed comparatively easily.
[0071] The adjustment of the positional relationship may be
realized by a different mechanism. For example, a mechanism for
moving the emitter tip 11 in directions of up, down, left and right
may be comprised so that the axis of the emitter tip 11 and the
center of the extraction electrode hole 131 are aligned. Also,
these adjusting mechanisms may be omitted.
[0072] Referring back to FIG. 1, the gas field ionization source 1
may comprise a vacuum gauge 18 for measuring a degree of vacuum in
the vacuum chamber 17. The gas pressure can be adjusted by
adjusting e.g. the flow rate adjusting means 32 automatically or
manually in response to the measured degree of vacuum. Appropriate
adjustment of the gas pressure reduces a ratio of the ion beam 15
being scattered by the ionization gas. For example, a gas pressure
in a region in a lower-pressure side with respect to the vacuum
partition 12 in the vacuum chamber 17 can be adjusted to be a
degree of vacuum equal to or higher than 0.1 Pa (i.e. a gas
pressure of less than 0.1 Pa).
[0073] Also, the degree of vacuum around the emitter tip 11 can be
obtained indirectly by monitoring the vacuum gauge 18. Control of
the gas pressure is very important because amount of electric
current emitted from the gas field ionization source 1 is
proportional to the gas pressure around the emitter tip 11. The
flow rate adjusting means 32 may be adjusted so that the gas
pressure in the vacuum chamber 17 is constant, or that fluctuation
of the gas pressure in the vacuum chamber 17 is suppressed, in
response to the degree of vacuum measured by the vacuum gauge
18.
[0074] If amount of the gas used is particularly large, the gas
stored in the gas supply means 31 decreases as the time passes, so
the gas pressure of the ionization gas supplied to the emitter tip
11 will eventually drop if a conductance of the flow rate adjusting
means 32 is constant. Such a phenomenon can be prevented by
adjusting the flow rate adjusting means 32 in response to the
degree of vacuum as described above.
[0075] By constructing the above gas field ionization source 1, an
effect of raising the gas pressure of the ionization gas locally
around the emitter tip 11 is obtained. Accordingly, a sufficient
intensity can be maintained for the ion beam 15 without cooling the
emitter tip 11 or with cooling weaker than conventional
constructions.
[0076] Also, the voltage required to generate an electric field
sufficient for ionizing the ionization gas at the tip end of the
emitter tip 11 (i.e. the electric potential difference between the
emitter tip 11 and the extraction electrode 13) can be an electric
potential difference of a fully practical scale (e.g. equal to or
less than 30 kV).
[0077] Further, it is constructed so that the gas supply means 31
is electrically connected to the emitter tip 11 or have an
electrical potential equal to the emitter tip 11, so there is no
risk of electric discharge via the ionization gas even if the gas
pressure is raised.
[0078] In addition, the adjusting mechanism for adjusting the
relative position between the emitter tip 11 and the hole 125 of
the vacuum partition 12 is comprised, so the relationship between
the pressure distribution of the ionization gas and the electric
field of the tip end of the emitter tip 11 can be adjusted to be
optimal.
[0079] In conventional techniques, the tip end of the emitter tip
may need to be sharpened to a range of about one to three atoms.
Although many methods have been proposed for sharpening to the
atomic level, there are problems that they are technically
difficult and monetary cost is high. Also, some methods have a
problem that steps for sharpening require time. In contrast,
according to the present embodiment, a sufficient ion beam
intensity can be obtained comparatively easily, so the tip end of
the emitter tip 11 does not have to be sharpened to the atomic
level. However, it is also possible to enhance performance further
by sharpening.
[0080] FIGS. 6A and 6B show an effect of an experimental example of
the present invention. The inventor confirmed an effect of the gas
field ionization source 1 related to the first embodiment by
actually creating a gas field ionization source comprising a
construction equivalent to the first embodiment and performing an
experiment for generating the ion beam.
[0081] FIG. 6A shows an evaluation result of ionization according
to a comparative example, i.e. a conventional gas field ionization
source. In the conventional construction, the image representing
the result detected by a detection plane 510 of the micro channel
plate for detecting ions is dark, indicating that the amount of ion
emission 511 from sample atoms is small.
[0082] FIG. 6B shows an evaluation result of ionization according
to a gas field ionization source related to the present experiment.
In this construction, the image representing the result detected by
a detection plane 520 of the micro channel plate 5 (FIG. 1) for
detecting ions is bright, indicating that the amount of ion
emission 521 from sample atoms is large.
[0083] Regarding the above two evaluations, detection conditions
for the micro channel plates are equivalent and the degrees of
vacuum measured by the vacuum gauge are also equivalent (and
therefore the degrees of vacuum in the paths that ion beams pass
are also equivalent). In other words, it can be said that the
degrees that the ion beams are scattered are similar for the two
results. However, there is a difference in the amounts of ion
emission from the samples, which means that the present experiment
example increased the electric current of the ion beam 15 by
enhancing the gas pressure only around the emitter tip 11.
According to the detected brightness in the micro channel plate,
the ion current related to the present experiment example is
increased to 7 to 20 times compared with the conventional ion
current.
[0084] Comparison between the gas field ionization source related
to the first embodiment and a conventional gas field ionization
source is further described below. In JP 2016-76431A, in order to
increase an intensity of the ion beam, an external electrode is
placed outside a container filled with ionization gas, electric
potentials of a needle-shaped electrode and an aperture electrode
around a micro aperture provided at the container to be equal, and
the gas is ionized by applying a strong electric field at the tip
end of the needle-shaped electrode by an electric potential
difference between the needle-shaped electrode, the aperture
electrode and the external electrode. It states that the gas is
ionized effectively by utilizing a steep pressure gradient
generated by the ionization gas ejecting from the micro aperture
and that probability of collision between the ions and the gas is
maintained low. Also, it states that this structure eliminates the
need for cooling and allows effective ionization with a simple
structure.
[0085] However, the method of JP 2016-76431A has a problem that it
is difficult to generate an electric field at the tip end of the
needle-shaped electrode because there is the aperture electrode
between the needle-shaped electrode and the external electrode, so
a very high voltage is applied between the needle-shaped electrode
and the external electrode. Also, the pressure gradient generated
around an outside of the aperture electrode is so steep that it is
very difficult to find an optimal relationship between the gas
introduction condition for enhancing ion source intensity and the
position of the needle-shaped electrode. Further, while it is
indispensable to introduce the ionization gas into the container at
as high a pressure as possible, the introduction gas pressure has
an upper limit because the ionization gas causes electric discharge
due to the electric potential difference between the container at a
high voltage and a gas introduction source (e.g. high pressure gas
cylinder) at the ground potential. The problem would be even more
difficult considering that the voltage applied between the
needle-shaped electrode and the external electrode is high.
[0086] In order to solve the above problem that it is difficult to
generate an electric field at the tip end of the needle-shaped
electrode, JP 2013-8471A uses the container made of a high
dielectic material instead of a metal in order to generate the
electric field between the high dielectic material and the external
electrode easily. Also, it states that, with further forming a
surface of the high dielectic material in the vicinity of the
needle-shaped electrode with a material that suppresses occurrence
of secondary electrons, a stable and high-intensity ion beam can be
generated continuously for a long period without degradation or
instability in the electric field strength in the vicinity of the
tip and of the needle-shaped electrode.
[0087] However, in the method of JP 2013-8471A, the high-dielectic
material is in the vicinity of the needle-shaped electrode, so the
electric field at the needle-shaped electrode cannot be controlled
from outside. Although degradation and instability in the electric
field strength in the vicinity of the tip end may be suppressed in
a stationary state, this is not the case if e.g. the emitted ion
beam current is changed, and there arises a problem that
re-adjustment is required for the external electrode or optical
elements such as an electric field lens in order to maintain
convergence performance for the ion beam. Also, there is a problem
that it takes time until the stationary state is reached again.
Further, there still are problems regarding the upper limit for the
high gas pressure due to electric discharge or optimization of the
condition for gas introduction and the position of the
needle-shaped electrode.
[0088] According to the gas field ionization source 1 related to
the first embodiment of the present invention, the problems
described above regarding JP 2016-76431A and JP 2013-8471A can be
avoided while making cooling of the emitter tip 11 unnecessary.
[0089] Following modifications can be made to the above first
embodiment. The ionization gas stored in the gas supply means 31
may be supplied after gas purification by using a gas filter or the
like if necessary. The gas filter may be provided before or after
the gas supply means 31 or both.
[0090] It can be constructed that an inside of the vacuum chamber
17 of the gas field ionization source 1 is maintained at an
ultrahigh vacuum of equal to or less than 10.sup.-7 Pa provided
that there is no gas introduction by the flow rate adjusting means
37. In order for the inside of the vacuum chamber 17 to reach the
ultrahigh vacuum, a startup operation of the gas field ionization
source 1 may include a process for heating the entire vacuum
chamber 17 to a temperature equal to or higher than 100 degrees
(referred to as "baking").
[0091] If the gas is introduced from the gas supply means 31 to an
entire inside of the vacuum chamber 17 at an excessively high gas
pressure, there will be a state wherein the gas pressure is high in
an entire optical path of the ion beam 15 emitted from the emitter
tip 11, leading to deficiency, e.g. a portion of the ion beam 15 is
scattered and beam convergence is deteriorated. In order to control
this appropriately, it is preferable that the gas pressure
introduced into the vacuum chamber 17 is around about 0.01 Pa.
[0092] Also, the pressure of the ionization gas around the emitter
tip 11 can be equal to or higher than 1 Pa.
[0093] The tip end of the emitter tip 11 may be sharpened. For
example, a method including heating of the emitter tip 11 may be
employed in order to terminate it with equal to or less than three
atoms, preferably with a single atom. Also, a method that
introduces a gas such as O.sub.2 or N.sub.2 into the vacuum chamber
17 during heating the emitter tip 11 may be employed in order to
sharpen the tip end of the emitter tip 11 in a similar manner.
Further, a method that introduces a gas such as O.sub.2 or N.sub.2
into the vacuum chamber 17 during applying a high voltage between
the emitter tip 11 and the extraction electrode 13 may be employed
in order to sharpen the tip end of the emitter tip 11 in a similar
manner. Further, a method that constructs the emitter tip 11 with a
needle including tungsten as a primary component, evaporates a
noble metal (e.g. iridium, palladium, platinum, etc.) on a surface
thereof and heats the emitter tip 11 in a similar manner.
[0094] Note that, although the present invention makes cooling of
the emitter tip 11 unnecessary or makes it operable with cooling
weaker than conventional, it does not exclude any construction
wherein the emitter tip 11 is cooled to a degree similar to a
conventional construction. The electric current of the ion beam 15
can be increased by cooling the emitter tip 11 if disadvantages of
cooling (additional cost or labor, or the like) can be tolerated in
accordance with applications or the like of the ion beam.
Second Embodiment
[0095] A second embodiment alters a construction for supplying the
ionization gas in the first embodiment. Hereinafter differences
with the first embodiment will be described.
[0096] FIG. 7 shows a construction of a gas field ionization source
2 related to the second embodiment. The gas field ionization source
2 comprises a gas supply means 45 instead of the gas supply means
31 and the gas cylinder 35 related to the first embodiment. Also,
the gas field ionization source 2 comprises a flow rate adjusting
means 43 instead of the flow rate adjusting means 32, the valve 33
and the flow rate adjusting means 37 related to the first
embodiment. The gas supply means 45 is not contained in the vacuum
chamber 17. Accordingly, the ionization gas can be supplied in a
single-stage construction, not in the two-stage construction as in
the first embodiment, so the construction is comparatively
simple.
[0097] The gas introduction tube 46 connects the gas supply means
45 and a space in the vicinity of the emitter tip 11. Electric
potentials of the gas supply means 45 and the emitter tip 11 can be
maintained to be equal by constructing the gas introduction tube 46
from a metal (or an electrically conductive material).
[0098] If the gas field ionization source 2 is operated, a high
voltage is applied to the gas supply means 45, so the gas supply
means 45 may be covered by a cover 42 made of a metal (or an
electrically conductive material) for safety. In other words, the
gas supply means 45 may be contained in the cover 42. Note that
"contain" here does not require that all directions are covered
completely. In the example of FIG. 7, a portion of periphery of the
gas supply means 45 is covered by the vacuum chamber 17, an
insulator 40 or an insulator 41 and another portion thereof is
covered by the cover 42.
[0099] The cover 42 may be connected electrically to the vacuum
chamber 17 or grounded. An inside of the cover 42 may be a vacuum
(e.g. of a degree similar to the vacuum chamber 17) or filled with
a gas which hardly discharges electrically (e.g. PF6). Also,
although FIG. 7 shows for clarity that there are gaps between the
vacuum chamber 17, the insulator 40 and 41 and the cover 42, they
may be joined with an airtightness practically sufficient.
[0100] The ionization gas is introduced from the gas supply means
45 to the vicinity of the emitter tip 11 via the flow rate
adjusting means 43. The electric potential of the flow rate
adjusting means 43 will be equal to that of the emitter tip 11, so
a ground-side adjusting mechanism 332 may be provided in order to
adjust the flow rate adjusting means 43. The ground-side adjusting
mechanism 332 and the flow rate adjusting means 43 are connected by
the connection mechanism 333.
[0101] The connection mechanism 333 may be a power transmission
mechanism or an information transmission means. If the connection
mechanism 333 is a power transmission mechanism, it can be
constructed by an electrically insulating member. If the connection
mechanism 333 is an information transmission means, it can be
constructed to communicate an instruction issued from the
ground-side adjusting mechanism 332 to the flow rate adjusting
means 43 in a remote manner by using an electric wave or the
like.
[0102] The gas supply means 45 may be constructed by a small gas
cylinder. Also, the gas supply means 45 may be provided with a
sealing valve 44 and may be constructed so that a gas cylinder for
charging can be connected from the outside via the sealing valve
44. In this construction, if the gas pressure within the gas supply
means 45 drops by usage of the ion source, the ionization gas can
be charged newly into the gas supply means 45 by removing the cover
42 and connecting the gas cylinder. After charging, the gas field
ionization source 2 can be used continuously by disconnecting the
charging gas cylinder and covering it with the cover 42 again.
DESCRIPTION OF SYMBOLS
[0103] 1, 2 Gas field ionization source
[0104] 5 Micro channel plate
[0105] 11 Emitter tip
[0106] 12 Vacuum partition
[0107] 13 Extraction electrode
[0108] 15 Ion beam
[0109] 16 Evacuation means
[0110] 17 Vacuum chamber (vacuum container)
[0111] 18 Vacuum gauge
[0112] 19 Bellows
[0113] 31 Gas supply means
[0114] 32 Flow rate adjusting means
[0115] 33 Valve
[0116] 34 Vacuum pump
[0117] 35 Gas cylinder (additional gas supply means)
[0118] 36 Valve
[0119] 37 Flow rate adjusting means
[0120] 38 Insulation tube
[0121] 40,41 Insulator
[0122] 42 Cover
[0123] 43 Flow rate adjusting means
[0124] 44 Sealing valve
[0125] 45 Gas supply means
[0126] 46 Gas introduction tube
[0127] 111,112 High voltage power source (means for applying
voltage)
[0128] 113 Emitter tip supporting component (adjusting
mechanism)
[0129] 114 Emitter tip positioning component (adjusting
mechanism)
[0130] 115a,115b Screw channel
[0131] 116 Rotation axis
[0132] 117 Protrusion of emitter tip
[0133] 121 Micro protrusion (convex structure)
[0134] 122 Height of micro protrusion
[0135] 123 External diameter of micro protrusion
[0136] 124 Distance between micro protrusion and extraction
electrode
[0137] 125 Hole of vacuum partition
[0138] 131 Extraction electrode hole
[0139] 132 Insulator
[0140] 330 Atmosphere-side adjusting mechanism
[0141] 331,333 Connection mechanism
[0142] 332 Ground-side adjusting mechanism
[0143] 510,520 Detection plane of micro channel plate
[0144] 511,521 Ion emission from atoms
* * * * *